Evaluation circuits of this kind are usually referred to as incremental position sensors. In them, the first signal and the second signal are usually referred to as the cosine signal and the sine signal. An approximate position, accurate to one signal cycle, is determined by evaluating the zero crossings of the signals. By also evaluating the values of the cosine and sine signal themselves—within one signal cycle—a precise position can be defined. The precise position emerges at
α = arctan (y′/x′)for x′ > 0α = π + arctan (y′/x′)for x′ < 0α = π/2 sign y′for x′ = 0.
x′ and y′ are signals which have been determined from the cosine signal and the x and the sine signal y (or the first signal x and the second signal y) through correction by offset, amplitude and phase errors 01, 02, A, φ. Ignoring other sources of error, the following appliesx=cos α−01y=A sin(α−φ)−02
The methods and circuits are generally known. The reader is referred, for example, to DE 100 19 500 A1 or EP 0 489 936 B1 of the applicant and to DE 195 02 399 A1.
Such evaluation methods and the corresponding evaluation circuits are used in particular for recording the position of rotary transducers. In them, one sensor unit in each case records the first analog signal or the second analog signal and feeds it to respectively to an AD converter. Each AD converter digitizes the analog signal fed to it with a scanning frequency and feeds a corresponding digital signal to an evaluation block. The evaluation block determines with the aid of the digital signals an arcus tangens corresponding to the digital signals as well as correction values for the first and the second signal.
Such an evaluation method is also known from the essay “Oversamplingverfahren zur Verbesserung der Erfassung von Lage and Drehzahl an elektrischen Antrieben mit inkrementellen Gebersystemen” [Oversampling methods for improving the recording of position and rotational speed in electric drives with incremental sensor systems] by Roland Kirchberger and Bernhard Hiller, both ISW Stuttgart. In this method, on which the present invention is based, the evaluation block is configured as a hardware circuit. The arcus tangens is determined in real time via a table filed in a memory. The scanning frequency of the AD converters is at least four times as large as the maximum frequency. In the circuit described in the technical essay, it lies at one megahertz.
The evaluation method of the technical essay and the corresponding evaluation circuit represent a considerable step forward compared with the previously mentioned conventional evaluation methods or evaluation circuits. However, the evaluation circuit needs a large storage requirement as the arcus tangens values are filed in the memory in the form of a look-up table.